Download Intercontinental Comparison of Fish Life History Strategies along a

American Fisheries Society Symposium 73:83–107, 2010
© 2010 by the American Fisheries Society
Intercontinental Comparison of Fish Life History Strategies
along a Gradient of Hydrologic Variability
Julian D. Olden*
School of Aquatic and Fishery Sciences, Box 355020, University of Washington
Seattle, Washington 98195, USA
Mark J. Kennard
Australian Rivers Institute, Griffith University, Nathan, Queensland 4111, Australia
Abstract.—The flow regime is considered the primary driver of physical processes
in riverine ecosystems; thus we expect that the trait composition of fish assemblages
might respond similarly to hydrologic variability, even at broad spatial scales. Here, we
test the hypothesis that freshwater fish life history strategies on two continents (southern United States and eastern Australia) converge along gradients of hydrologic variability and primary productivity at the drainage scale. Our results show that the fishes
of the United States and Australia conform to the three-dimensional adaptive space
arising from the trade-offs among three basic demographic parameters of survival,
fecundity, and onset and duration of reproductive life. Species from both continents
represent the endpoints in adaptive space defining the periodic (19% versus 33% for
the United States and Australia, respectively), opportunistic (69% versus 52%), and
equilibrium life history strategies (12% versus 15%). We found evidence that fish life
history composition of drainage basins in the two continents have converged across
similar gradients of hydrologic variability and productivity despite phylogenetic and
historical differences. Moreover, these relationships were largely consistent with predictions from life history theory. Increasing hydrologic variability has promoted the
greater prevalence of opportunistic strategists (a strategy that should maximize fitness in environmental settings dominated by unpredictable environmental change)
while concurrently minimizing the persistence of periodic-type species (a strategy
typically inhabits seasonal, periodically suitable environments). Our study provides
a conceptual framework of management options for species in regulated rivers because life history strategies are the underlying determinants for population responses
to environmental change and therefore can be used to classify typical population responses to flow alteration or mitigation via environmental flow prescriptions.
Introduction
nized (Naiman et al. 2008). Flow has been
suggested to be the “master variable” that determines pattern and process in rivers, thus limiting the distribution and abundance of species
and regulating ecological integrity (Poff et al.
1997). Natural spatial variation in the hydro-
The importance of hydrologic variability for
shaping the biophysical attributes and functioning of riverine ecosystems is well recog* Corresponding author: [email protected]
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olden and kennard
logic regime is influenced by variations in climate and basin geology, topography, and vegetation, which interact at multiple spatial and
temporal scales to shape the physical template
upon which ecological and evolutionary processes operate in river ecosystems (Poff and
Ward 1990; Bunn and Arthington 2002; Lake
2008).
The natural flow-regime paradigm postulates that the structure and function of riverine
ecosystems, and the adaptations of their constituent riparian and aquatic species, are dictated by patterns of intra- and interannual variation in river flows (Poff et al. 1997). A rich body
of literature has demonstrated that the longterm physical characteristics of flow variability
have strong consequences at local to regional
scales and at time intervals ranging from days
(ecological effects) to millennia (evolutionary effects). Flow variability, including flood
and drought events, interacts with the underlying geology to shape the river’s physical and
chemical templates and constrain assemblage
structure for fish (e.g., Lamouroux et al. 2002;
Hoeinghaus et al. 2007; Kennard et al. 2007),
stream invertebrates (e.g., Poff and Ward 1989;
Vieira et al. 2004; Dewson et al. 2007), riparian plants (e.g., Nilsson et al. 1993; Naiman and
Décamps 1997; Pettit et al. 2001), and riparian
invertebrates (e.g., Wenninger and Fagan 2000;
Lambeets et al. 2008). Adaptations to natural
flow regimes include behaviors that enable insects to avoid desiccation by droughts, fish life
history strategies that are synchronized to take
advantage of floodplain inundation, and plant
morphologies that protect roots by jettisoning
seasonal biomass during floods (reviewed in
Lytle and Poff 2004).
There has been considerable interest in the
identification of major axes of ecological strategy variation in freshwater ecosystems based
on trait correlations across large numbers of
species (Winemiller 2005; Olden et al. 2006;
Poff et al. 2006; Verberk et al. 2008; Frimpong
and Angermeier 2010, this volume). Species
traits may be intercorrelated through physiological constraints, trade-offs (i.e., investments
in one trait leaving fewer resources available
for investment in another), or spin-offs (i.e., investments in one trait reduce costs or increase
the benefits of investment in another trait),
resulting in the creation of life history strategies or tactics represented as sets of coevolved
traits that enable a species to cope with a range
of ecological problems (Stearns 1992). Comparative studies from a diverse array of fishes
in marine and freshwater systems have independently identified three primary life history strategies that represent the endpoints of
a triangular continuum arising from essential
trade-offs among the basic demographic parameters of survival, fecundity, and onset and
duration of reproduction (Winemiller 1989,
1992; Winemiller and Rose 1992; Vila-Gispert
and Moreno-Amich 2002; Vila-Gispert et al.
2002; King and McFarlane 2003; see Figure
1). Winemiller and Rose (1992) synthesized
these life history trade-offs and proposed the
following characteristic biological and habitat
environmental attributes associated with the
three life history strategies. Periodic strategists
are large-bodied fishes with late maturation,
high fecundity per spawning event, and low juvenile survivorship (i.e., no parental care) and
typically inhabit seasonal, periodically suitable
environments with large-space spatial (patchiness) and temporal (seasonality) heterogeneity. Opportunistic strategists are small-bodied
fishes with early maturation, low fecundity per
spawning event, and low juvenile survivorship
and typically inhabit habitats subjected to frequent and intense disturbances. Equilibrium
strategists are small to medium-bodied fishes
with moderate maturation age, low fecundity
per spawning event, and high juvenile survivorship (i.e., provides parental care) and typically
intercontinental comparison of fish life histories
85
Figure 1.­Triangular life history model depicting environmental gradients selecting for endpoint strategies defined by optimization of demographic parameters generation time, fecundity, and juvenile survivorship (modified from Winemiller 2005). Example species from the study regions and representative
hydrologic regimes expected to favor each life history strategy are illustrated. Australian fish illustrations
by B. J. Pusey; U.S. fish illustrations freely available on the Internet.
inhabit environments with low variation in
habitat quality and strong biotic interactions.
The demographic parameters discussed
above are direct reflections of the ways in
which fish allocate energy to reproduction,
and the three life history strategies of the continuum can be interpreted as being adaptive
with respect to relative variability and predictability of temporal and spatial variation in abiotic environmental conditions, food availability, and predation pressure (Winemiller 2005).
Hydrological variability plays a dominant role
in shaping physical processes in riverine ecosystems, and a number of recent studies have
supported the association between hydrology
and fish life history strategies. Tedesco and
Hugueny (2006) found that regional climate
(presumably related to hydrologic variability)
induced greater population synchrony of peri-
odic fish species in rivers of the Cote d’Ivoire
(Africa) compared to equilibrium species. By
comparing trends in native and nonnative species distributions among fish life history strategies in the lower Colorado River basin (USA),
Olden et al. (2006) found that century-long
modifications in flow variability have likely
promoted the spread of nonnative equilibrium
strategies (favored in constant environments)
while leading to greater distributional declines
of native species located along the periodicopportunistic continuum (strategies favored
in more unpredictable and variable environments). Tedesco et al. (2008) found higher
proportions of periodic species in highly seasonal drainage basins of western Africa (e.g.,
rivers with short and predictably favorable
seasons), whereas more hydrologically stable
basins with a wet season of several months
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olden and kennard
were dominated by equilibrium strategists. Collectively, these studies (and others) suggest that
the distributions of freshwater fishes are shaped,
at least in part, by interactions between life history strategies and patterns of predictability and
variability in hydrologic regimes.
Enhancing our mechanistic understanding of the functional linkages between environmental drivers of fish species distributions
will provide a foundation for predicting existing and future impacts of altered flow regimes,
invasive species, and land-use change. Gaining
this knowledge is predicated upon examining
how fish assemblage structure varies in response to flow variability and testing the generality of these relationships across large spatial
scales. By using the “habitat templet” concept
proposed by Southwood (1977), and applied
to riverine systems by Townsend and Hildrew
(1994), we predict that because life history
strategies are synchronized with long-term hydrologic dynamics, even geographically distant
regions will presumably favor the persistence
of species with similar traits if subjected to similar historical flow regimes. If the functional
characteristics of organisms and communities
are predictable from features of their environment, then convergence in these characteristics across regions implies the existence of key,
repeated evolutionary mechanisms responsible for these relationships (Schluter 1986).
Community convergence driven by similar
environmental selective forces would be supported by observations of similar relationships
between spatial variations in community life
history composition along environmental gradients (Lytle and Poff 2004).
In the present study, we explore this question by providing an intercontinental examination of patterns in fish functional composition
across a gradient of hydrologic variability and
productivity in the southern United States
and eastern Australia, focusing on the oppor-
tunistic-periodic-equilibrium trichotomy. The
freshwater fish faunas differ strikingly between
the two regions, with the U.S. fauna being extremely diverse and with low endemism in
comparison to the comparatively depauperate
and highly endemic Australian fauna. Our primary objective was to test the hypothesis that
the relationship between fish life history patterns, hydrologic variability and net primary
productivity (NPP) in the United States and
Australia is concordant with predictions from
life history theory, as proposed by Winemiller
(1995, 2005) and McCann (1998) (Figure 1).
Specifically, by virtue of their small size and
rapid turnover rates, we predict that opportunistic species should maximize fitness (and
thus be more prevalent) in drainage basins
characterized by productive and frequently
disturbed habitats (i.e., high hydrologic variability and high NPP). By contrast, periodic
and equilibrium strategists are predicted to be
more common in drainage basins exhibiting
low-flow variability and either more productive habitats for periodic strategists (i.e., low
hydrologic variability and high NPP) or less
productive habitats for equilibrium strategists
(i.e., low hydrologic variability and low NPP).
For periodic strategists this may be seen as a
consequence of the storage effect (i.e., long life
stage buffers times of low recruitment: Warner
and Chesson 1985) and high annual fecundities, and for equilibrium strategies as a result
of low adult mortality rates that enable them to
survive on lower resource biomass (McCann
1998). By taking a functional approach, our
study underscores the theoretical expectation
that species traits promoting local persistence
will change along environmental gradients,
thus facilitating the comparison of regions
separated by large geographic distances and
aiding the development of broadly applicable
generalizations regarding patterns of fish life
history strategies. If fish species show similar
intercontinental comparison of fish life histories
ecological responses to the same environmental challenges irrespective of their ancestry, we
expect that hydrologic variability has selected
for similar community patterns in fish life history strategies despite originating from completely different regional species pools at opposite sides of the world.
Methods
Study Region
We used an updated global map of the KöppenGeiger climate classification (Peel et al. 2007) to
identify study regions in the United States and
Australia, having experienced similar regional
climate (average annual air temperature and total precipitation) and runoff regimes. By selecting study regions from the same climate type, we
were facilitating the direct comparison of fish assemblages from drainage basins that have likely
experienced a similar range of historical climatic
and runoff conditions while still exhibiting inter-
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basin differences (see below). We selected drainage basins in the United States (n = 56 basins)
and Australia (n = 56 basins) that are located
within the temperate climate types Cfa and Cfb,
representing the most prevalent climate that is
common to both continents (Figure 2A). These
climate regions are characterized by a temperate
climate, without a dry season, and experience
either a hot summer with the hottest month
$228C (Cfa) or a warm summer with the hottest month less than 228C and $ 4 months
where temperatures exceed 108C (Cfb). All
drainage basins in the United States discharge to
the Gulf of Mexico and are located in the Ohio
River, Tennessee River, lower Mississippi River,
and coastal regions of southeastern states, as
well as including small areas of the upper Mississippi, Missouri, Arkansas, White, and Red
rivers (Figure 2B). Drainage basins in Australia
include all major eastern coastal rivers between
the Fitzroy River in the north and the Tarwin
River in the south (Figure 2C).
Figure 2. (A) Global map of the Köppen-Geiger climate classification (from Peel et al. 2007) depicting
the two study regions in the United States and Australia (squares) that are located within the temperate climate types Cfa and Cfb. (B) Fish species richness of drainage basins in the United States. (C) Fish
species richness of drainage basins in Australia.
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Species Distributions
We assembled present-day species lists of native freshwater fishes (i.e., excluding nonnative
translocated native species) for drainage basins
located in our study region (hereafter termed
the “United States” and “Australia” for brevity,
although specifically referring to the southern United States and eastern Australia). We
defined a freshwater fish in a relatively broad
sense to include those species that can reproduce in freshwater and diadromous species
that spend the majority of their lives in freshwaters. We excluded numerous species with
strong marine or estuarine affinities that may
enter freshwaters for only short periods of time.
United States distribution data on 559 native
fish species (from 36 families and 104 genera)
in 56 drainage basins were obtained from the
NatureServe Database (NatureServe 2004),
which summarizes data compiled by state
natural heritage programs, museum records,
published literature, and expert opinion. The
database was originally compiled at the 8-digit
hydrologic unit code (representing a hierarchical system of drainages) but was converted to
the four-digit hydrologic unit code scale to ensure the completeness of species records. Native freshwater fish species distribution data
for Australia was assembled using information
from Pusey et al. (2004), as well as by reference
to data sets held by state government agencies,
regional experts, existing literature, and interrogation of museum records. Fish occurrence
data for Australia was obtained for 66 native
fish species (from 25 families and 45 genera)
in 56 drainage basins.
Species Traits
We used the scientific literature and electronic databases to provide a comprehensive
functional description of the native freshwater fishes of the study regions in the United
States and Australia. We collated data for 14
ecological and life history attributes that could
be justified on the basis of our current state
of knowledge and information available for
the majority of species pools (Table 1). These
traits were divided into five categories according to body morphology—maximum total
body length, shape factor, and swim factor
(following Webb 1984); behavior—substrate
preference and vertical position; life history—
longevity, age at maturation (female), length at
maturation (female), total fecundity, egg size,
spawning frequency, reproductive guild (following Balon 1975), and parental care (following Winemiller 1989); and trophic—trophic
feeding guild according to adult feeding mode
based on published diet analyses. Parental care
was quantified as the Sxi for i = 1–3, where x1
= 0 if no special placement of zygotes, x1 = 1
if special placement of zygotes, x1 = 2 if both
zygotes and larvae maintained in nest, x2 = 0 if
no parental protection of zygotes or larvae, x2
= 1 if brief period of protection by one sex (<1
month), x2 = 2 if long period of protection by
one sex (>1 month) or brief care by both sexes,
x2 = 4 or lengthy protection by both sexes (>1
month), x3 = 0 if no nutritional contribution to
larvae, x3 = 2 if brief period of nutritional contribution to larvae (<1 month), x3 = 4 if long
period of nutritional contribution to larvae
(1–2 months), and x3 = 8 if extremely long period of nutritional contribution to larvae (>2
months).
Trait assignments were based on a multitiered data collection procedure. First, trait
data were collected from species accounts in
the comprehensive texts of the regional fish faunas: United States (references available upon
request) and Australia (Pusey et al. 2004 and
references therein). Second, we used species
descriptions from the primary literature, state
agency reports, university reports, and graduate theses. Third, we obtained data from elec-
intercontinental comparison of fish life histories
89
Table 1. Ecological and life-history traits quantified for freshwater fishes of the United States and
Australia.
Trait
Description
Abbreviation
Body morphology
Maximum body length Maximum total body length (cm)
Shape factor
Ratio of total body length to maximum body depth
Swim factor Ratio of minimum depth of the caudal peduncle to the maximum body depth Behavior
Substrate preference
Coarse (rocks, cobble, coarse gravel)
Fine (sand, fine gravel)
Silt/mud General
Vertical position
Benthic Nonbenthic Life history
Longevity
Maximum potential life span (years)
Age at maturation Mean age at maturation (years)
Length at maturation
Mean total length at maturation (cm)
Total Fecundity
Total number of eggs or offspring per breeding season Egg size
Mean diameter of mature (fully yolked) ovarian oocytes (mm)
Spawning frequency
Single spawning per lifetime
Single spawning per season
Multiple (batch/repeat/protracted) spawning per season
Reproductive guild
Nonguarders (open substratum spawners)
Nonguarders (brood hiders)
Guarders (substratum choosers)
Guarders (nest spawners)
Bearers (internal)
Bearers (external)
Parental care
Metric representing the total energetic contribution of parents to their offspring sensu Winemiller (1989)
Trophic
Trophic guild
Herbivore-detritivore (ca. >25% plant matter)
Omnivore (ca. 5–25% plant matter)
Invertivore
Invertivore–piscivore
tronic databases available on the World Wide
Web, including FishBase, and U.S. state natural
heritage programs. Fourth, expert knowledge
was used to assign values to a small number
of trait states that could not be obtained from
the previous methods (mainly inferred from
congenerics). Trait values were represented by
ordinal, nominal, or continuous data (Table
1). Ordinal and nominal traits were assigned
a single state based on a majority of evidence
rule according to adult preferences, and median values for continuous traits were used when
ranges were presented. Trait values for age and
MBL
ShapeF
SwimF
Coa
Fin
Sil
Gen
Ben
Non-Ben
Long
MatAge
MatLen
Fecund
EggS
SinL
SinS
MulS
NgOS
NgBH
GSC
GNS
BI
BE
ParentC
HeDe
Omni
Inve
InvePisc
length at maturation were recorded for female
specimens when possible.
Environmental Variables
Basin-level life history composition was modeled using contemporary environmental factors selected a priori, based on previous empirical demonstrations of their importance as
determinants of global patterns of fish species
richness (e.g., Oberdorff et al. 1997; Guégan et
al. 1998). These variables included total surface area of the drainage basin (km2) and mean
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annual runoff (million liters per square kilometer: ML/km2) as measures of habitat availability and diversity, net terrestrial primary
productivity (NPP, metric tons of carbon per
km2) as a measure of measure of available energy, and coefficient of variation in annual runoff
as a measure of hydrologic variability. Net terrestrial primary productivity was used because
net aquatic primary productivity was not available. Notably, by only selecting drainage basins
that are located in the same climatic region,
we eliminated the need to include variables
such as latitude, air temperature, and annual
precipitation to represent broad-scale climate
conditions. Annual NPP was estimated using
the Carnegie Ames Stanford Approach carbon
model (the amount of carbon produced by
ecosystems per 0.25 degree grid cell), where
the average value of all grid cells in each basin
was computed using a geographic information
system. The NPP data set is distributed by the
Columbia University Center for International
Earth Science Information Network and was
obtained from Imhoff et al. (2004). Briefly, the
model incorporates satellite and climate data
(1982–1998) to estimate the fixation and release of carbon based on a spatially and temporally resolved prediction of NPP in a steady
state (described in detail by Imhoff and Bounoua 2006). We chose to apply a global model
to estimate NPP rather than continental models that are readily available for both the United States (MODIS: Terra Dynamic Simulation
Group) and Australia (Australian Natural Resources Data Library: CSIRO) to control for
inherent differences in NPP predictions arising from differences in model structure.
On an interannual basis, variability in basin runoff will act as a qualifier of disturbance
and habitat availability and suitability in riverine ecosystems. We selected drainage basins
from the United States and Australia located in
same temperate climate type (Cfa and Cfb) to
ensure a similar historical range of flow conditions but potentially some level of interbasin
differences in hydrologic variability within
and between study regions. Also, note that
data availability precluded the quantification
of intra-annual (seasonal) variability in discharge. However, we note that at the continental scale, there is a strong correlation between
inter-annual variability (used in this study) and
intra-annual variability in discharge: United
States (R = 0.65, P < 0.05, n = 420, data source:
Olden and Poff 2003) and Australia (R = 0.78,
P < 0.05, n = 830, data source: Kennard et al.,
in press). Below, we discuss the methodology
used to quantify basin runoff and hydrologic
variability.
For the United States, U.S. Geological Survey daily streamflow data for 1900–2002 were
used to estimate runoff (streamflow per unit
area) for the hydrologic cataloging units in the
study region, following the approach of Krug
et al. (1987). This data set is available from
the U.S. Geological Survey (http://water.usgs.
gov/waterwatch) and was created by first identifying stream gauges with the following characteristics: (1) complete data for each water
year, (2) drainage basins contained entirely
within a cataloging unit, and (3) drainage areas
that did not overlap with the drainage basins
of other gauges in the same cataloging unit.
The average daily flow for the water year then
was divided by the drainage basin area for each
gauge to calculate runoff, and a drainage basin
area weighted average runoff value was computed for each cataloging unit. An adequate
number of gauges were identified using the
above criteria only for some cataloging units
during some years. For the remaining cataloging units, the runoff estimates were based on
(1) gauges with small drainage basins with an
unknown degree of overlap, (2) runoff values
for neighboring cataloging units, and (3) average runoff values for larger accounting units.
intercontinental comparison of fish life histories
We refer the reader to Krug et al. (1987) for
more details.
For Australia, drainage basin runoff was
estimated using discharge data modeled using
the water balance module of the GROWEST
program (Hutchinson et al. 2004). GROWEST operates on a weekly time step, converting
monthly input rainfall and evaporation data to
weekly values via cubic Bessel interpolation.
The water balance module is conceptualized
as a single “bucket” model. It adds rainfall to
the previous soil storage and removes it by
means of evapotranspiration. The soil water
surplus or “runoff ” is the rainfall exceeding
“bucketful” after allowing for evapotranspiration. Monthly rainfall and pan evaporation estimates were generated at a grid spacing of 0.01
degrees (approximately 1 km) using elevation
values derived by resampling the 9 s DEM version 3 with bilinear interpolation and monthly
climate surface coefficients for a 30-year period from 1971 to 2000. We refer the reader to
Stein et al. (2008) for more details. Monthly
and annual catchment water balance values
were calculated from the accumulated totals of
the upstream grid cell runoff estimates for each
catchment. Although we recognize that differences exist in how runoff was modeled in the
United States and Australia, the spatial resolution of global runoff models are too coarse
to provide accurate estimates of runoff at the
watershed scale.
The range of variation in mean annual
runoff (USA: 19.5–698.1 ML/km2, AUS:
23.3–734.2 ML/km) and NPP measured
(USA: 2.6–10.7 metric tons of carbon/km2,
AUS: 2.9–10.1 metric tons of carbon/km2)
was similar for both regions, although drainage
basins in the United States tended to be larger
(mean = 35,584 km2, range = 9,453–84,433
km2) than in Australia (mean = 6,727 km2,
range = 110–141,281 km2), and runoff in Australia was more variable (mean = 1.07, range =
91
0.45–2.10) than in the United States (mean =
0.56, range = 0.21–1.81).
Data Analyses
Accounting for phylogeny.—It is expected
that species share similar life history attributes through descent from common ancestry (Fisher and Owens 2004). It is therefore
necessary to account for phylogenetic effects
when exploring patterns in ecological data
because nonindependence among species
violates the assumption of standard statistics
that errors are uncorrelated and may result in
biased parameter estimates and increased type
I error rates if phylogeny is not taken into account. Phylogenetically informed analyses
require an estimate of the phylogenetic relationships among the taxa concerned. Current
approaches for controlling the effects of phylogeny typically involve the method of independent contrasts (Felsenstein 1985); however, this technique cannot accommodate mixed
variables types that are present in our data set
and requires a complete phylogeny (i.e., estimates of the branch lengths associated with
the phylogenetic tree) that is currently not
available. Therefore, we estimated the degree
of phylogenetic relatedness following Tedesco
et al. (2008), where all species within the same
genus were assigned a value of 1.0, all species
within the same family a value of 0.5, all species within the same order a value of 0.33, and
all species from different orders a value of 0.25.
The resulting phylogenetic similarity matrix
was then converted to a phylogenetic dissimilarity matrix (i.e., 1 similarity) and used to account for phylogeny in subsequent analyses.
Similarities in species’ trait characteristics.—
We summarized similarities in species’ trait
characteristics by conducting a principal coordinate analysis (PCoA) on the species-by-trait
matrix. PCoA is a statistical methodology to
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olden and kennard
explore and to visualize similarities/dissimilarities in multivariate data by optimally representing the variability of a multidimensional
data matrix (here, the species-by-trait matrix)
in ordination space with reduced (i.e., lower)
dimensionality. A species similarity matrix
according to the 14 biological traits was calculated using Gower’s similarity coefficient, a
metric able to accommodate mixed data types
and missing values (Legendre and Legendre
1998). These properties make Gower’s coefficient an appropriate choice for our analysis
given that the data set contains nominal, ordinal, and continuous traits and that it was not
possible to assign values for all traits to all species given the lack of scientific knowledge (Table 1). Prior to conducting a PCoA, we used
a modified version of the eigenvector method
presented by Diniz-Filho et al. (1998) and applied by Olden et al. (2006) to partition the
total variance in the biological trait distance
matrix into its phylogenetic and specific components using a Mantel test. We regressed the
phylogenetic distance matrix (see Accounting
for phylogeny) against the trait distance matrix
(based on Gower’s similarity) using a Mantel
test to derive a residual matrix that represents
trait similarities among species after controlling for phylogenetic relatedness. The Mantel
test showed a low correlation between the trait
and phylogenetic distance matrices (Mantel’s
standardized R = 0.279, P = 0.244), indicating
only a marginal degree of phylogenetic constraint. PCoA was then performed on the residual similarity matrix to summarize the dominant patterns of variation among the biological
traits and examine functional similarities and
differences among species. Only the first two
principal components were statistically significant (P < 0.05, based on the broken-stick rule:
Legendre and Legendre 1998), and they were
used to facilitate visual interpretation of the resulting plots.
Species life history strategies.—We examined the life history strategies of 257 fish species in the United States (limited to those species in which complete life history trait values
were available but representative of the entire
pool of U.S. species) and 66 fish species in Australia. We employed two approaches to quantify the community composition of basins
with respect to the opportunistic, periodic, or
equilibrium strategies. This analysis did not account for phylogeny, given its weak influence
on patterns of life history trait variation (see
result above).
First, we evaluated the fish life history
continuum model of Winemiller and Rose
(1992) by plotting species’ positions in relation to three life history axes: (1) ln maturation size (a surrogate of maturation age that is
highly correlated with maturation size in our
study, Pearson’s R = 0.82 across all species);
(2) ln mean fecundity; and (3) investment
per progeny, calculated as ln(egg diameter +
1) + ln(parental care + 1). Each species was
assigned to one of the three life history strategies (opportunistic, periodic, or equilibrium)
by calculating the Euclidean distance in trivariate life history space between the species’ position and the strategy endpoints and
designating the species to the closest strategy.
Strategy endpoints where defined as the following: opportunistic (minimum fecundity,
minimum juvenile investment, and minimum maturation size), periodic (maximum
fecundity, minimum juvenile investment, and
maximum maturation size), and equilibrium
(mean fecundity, maximum juvenile investment, and maximum maturation size). This
calculation was based on normalized trait values (i.e., standardized range between 0 and 1
for each trait) to ensure equal contributions
of the three life history parameters. This approach represents an advance over past studies that have assigned species to life history
intercontinental comparison of fish life histories
strategies according to arbitrary, but ecologically relevant, thresholds (i.e., Tedesco et al.
2006; Hoeinghaus et al. 2007). Basin-level
life history composition was summarized as
the proportional species richness for each life
history strategy.
Second, we recognize that many species
occupy immediate positions in life history
space, thus questioning the validity of a strict
classification of species into the opportunistperiodic-equilibrium trichotomy. We addressed this issue by computing a distanceweighted measure of proportional species
richness for each life history strategy. This
was accomplished by normalizing the Euclidean distances in trivariate life history space
between species and each strategy endpoint
(i.e., standardized range between 0 and 1
for each distance), computing the inverse of
these values (so that larger values represent
a greater “weight” or affinity of a species toward a life history strategy) and summing
the weights for each strategy in each basin
based on the species present. Basin-level life
history composition was summarized as the
proportional composition of each life history
strategy. In essence, this method produces
distance-weighted proportional species richness for each life history strategy.
Last, we summarized patterns of life history diversity by conducting a principal component analyses (PCA) on seven life history
traits that included maximum body size, longevity, length at maturation, maturation age,
fecundity, egg size, and parental care (Table
1). All traits are continuous and were log(x +
1)-transformed prior to the analysis to account
for heteroscedasticity. The resulting ordination
summarizes life history similarities among fish
species and enabled the identification of a reduced set of axes that best separated species
along the opportunistic-periodic-equilibrium
gradient.
93
Relationships of basin life history composition and environmental variables.—We used
multiple linear regression analysis to model
basin-level life history composition (summarized as the distance-weighted proportional strategy richness: see Species life history strategies) as a function of drainage basin
area, mean annual runoff, mean NPP, and
hydrologic variability. Analyses were conducted separately for the United States and
Australia, and basin area, runoff, and NPP
were log-transformed prior to analysis. Following Schluter (1986) and Lamouroux et
al. (2002), basin-scale convergence was analyzed by comparing the effects of hydrologic
variability and productivity on fish life history
traits, where a similar effect (based on direction and significance) of these factors within
both continents indicates convergence.
Results
Trait Composition of U.S. and Australian
Fishes
Freshwater fishes from the study regions in
United States and Australia varied in their
morphological, ecological, and life history attributes. The first two principal components
of the PCoA, after controlling for phylogeny,
explained 35.6% of the total variation represented by the 14 traits; additional axes were
nonsignificant and did not alter the interpretation of results. The first principal axis
identified a trait gradient that generally contrasted Australian fishes occurring mostly in
the right-hand side of the ordination from the
U.S. fishes that occupied the entire trait space
(Figure 3). This axis describes a morphological and life history gradient, with large-bodied, long-lived, late maturing, highly fecund
species with low parental care at high values
and smaller-bodied, short-lived, early maturing, low fecund species with high parental
94
olden and kennard
Figure 3. Two-dimensional ordination plot resulting from the principal coordinate analysis on the 14
biological traits for the fish species pools of the United States and Australia. (A) Fish species biplot
where solid symbols represent the United States and open symbols represent Australia. (B) Eigenvector
plot of the traits with the highest combined loadings (>0.50) on the first two principal components. Full
descriptions of trait abbreviations are in Table 1.
care at low values. Axis I also separated species with generalist feeding behaviors (omnivore, insectivore-piscivore) and low swim
factors (i.e., stronger swimming ability) in
positive ordination space from specialist invertivores with high swim factors (i.e., weaker
swimming ability) in negative ordination
space (Figure 3). While notable differences
in continental species pools do exist, a large
number of species from both the United
States and Australia are located in positive
axis I space, thus exhibiting trait similarities
despite lacking a common ancestry (Figure
3). Other important discriminators of trait
variation among species included vertical position (benthic versus nonbenthic), spawning
frequency (single versus multiple spawning
events per season), and substrate preference
(coarse versus fine grain).
Univariate trait comparisons support
intercontinental comparison of fish life histories
95
Figure 4. Univariate comparisons of the fish species pools in the United States (black bars) and Australia
(gray bars) according to (A) maximum body length, (B) swim factor, (C) total fecundity, (D) relative
maturation (age of maturity/longevity), (E) reproductive guild, (F) parental care, and (G) trophic guild.
continental patterns revealed in the multivariate analyses. The United States contained
a greater proportion of fish species that were
smaller-bodied (Figure 4A), exhibit lower
fecundity (Figure 4C), and mature at a rela-
tively older age (calculated as age of maturity
divided by longevity) compared to the Australian fish species pool (Figure 4D). Australian fishes showed a slight trend toward
exhibiting lower swim factors compared to
96
olden and kennard
fishes of the United States (Figure 4B). The
majority of United States fishes (>70%) either hide or guard their eggs during reproduction, whereas more than 65% of Australian
species spawn on open substrate and exhibit
no egg guarding (Figure 4E), a pattern also
reflected in the relatively greater amount of
parental care afforded by fish species of the
United States compared to Australia (Figure
4F). Only small differences were observed
for trophic guilds, with Australia containing
relatively more piscivorous species and the
United States supporting a relatively greater
number of invertivores (Figure 4G).
Life History Strategies of U.S. and
Australian fishes
Australian and United States freshwater fishes
conformed to the life history continuum model defined by the demographic parameters of
survival, fecundity, and onset and duration of
reproduction (Figure 5A). We found strong
evidence for the triangular adaptive surface
Figure 5. Life history diversity of freshwater fishes from southern United States (solid triangles) and
eastern Australia (open triangles). (A) Species are located on a two-dimensional triangular surface,
embedded in the three-dimensional space established by fecundity (ln scale), length at maturity (ln
scale) and juvenile survivorship (equal to ln (egg diameter + 1) + ln (parental care + 1)). The vertices
of this triangular surface define three endpoint strategies: opportunistic, periodic, and equilibrium;
and intermediate strategies are recognized within the gradient of life histories. (B) and (C) illustrate
a three-dimensional linear plane fitted to all species (indicated by circles).
intercontinental comparison of fish life histories
anchored by the opportunistic, periodic, and
equilibrium life history strategies (Figure 5B,
5C), where a two-dimensional linear plane
showed a good fit to fish species of Australia
(Pearson’s R = 0.65, P < 0.001, n = 66), the
United States (R = 0.76, P < 0.001, n = 257),
and both countries pooled (R = 0.71, P < 0.001,
n = 323). For the United States, a conspicuous
cluster of species were located in close proximity to the opportunistic endpoint, although all
endpoint and intermediate strategies were well
represented. By contrast, the majority of Australian fishes occupied positions in life history
space along a linear axis connecting the opportunistic and periodic endpoints (i.e., fewer species with equilibrium characteristics).
Species from both the United States and
Australia occupied the extreme opportunistic,
periodic, and equilibrium endpoints, although
the majority of species take up intermediate
positions in life history space. From the United
States, examples of more opportunistic strategies include select species of shiners (Cyprinella spp. and Notropis spp., Family: Cyprinidae)
and topminnows (Fundulus spp., Family: Fundulidae); periodic strategists include suckers
(Ictiobus spp. and Catostomus spp., Family: Catostomidae) and pickerel (Esox spp., Family:
Esocidae); and equilibrium strategists include
black basses (Micropterus spp., Family: Centrarchidae) and catfishes (Ameiurus spp. and
Noturus spp., Family: Ictaluridae). From Australia, examples of opportunistic strategists include select species of gudgeons (Hypseleotris
spp., Family: Eleotridae) and rainbowfishes
(Melanotaenia spp., Family: Melanotaeniidae);
periodic strategists include basses (Macquaria
spp., Family: Percichthyidae) and grunters
(leathery grunter Scortum hillii and barred
grunter Amniataba percoides, Family: Terapontidae); and equilibrium strategists include
lungfish (Australian lungfish Neoceratodus forsteri, Family: Ceratodontidae) and catfishes
97
(dewfish Tandanus tandanus, Family: Plotosidae; Arius graeffei, Family: Ariidae). Despite
these examples, the large majority of species
exhibited immediate positions in life history
space.
Multivariate ordination of fish species according to their life history attributes identified
two major gradients of trait variation represented by the first two PCA axes that explained
81.5% of the variance in the original seven life
history characteristics (Figure 6). The first
principal component revealed a dichotomy
between periodic strategists (i.e., relatively
larger species with a long life span, late [and
old] maturation, and high fecundity) occurring on the positive scale, from opportunistic
strategists (i.e., relatively smaller species with a
short life span, early [and young] maturation,
and low fecundity) occurring on the negative
scale. The second principal component identified a dominant gradient of life history traits
that contrasts species exhibiting high parental care, large egg size, and low fecundity with
positive scores from species showing minimal
parent care, small egg size, and relatively higher
fecundity with negative scores. This represents
a dichotomy between species considered equilibrium strategists versus periodic and opportunistic strategists (Figure 6).
Basin Life History Composition along a
Hydrologic and Productivity Gradient
Drainage basins of the United States and Australia showed similarity in their composition
of opportunistic life history strategies, on average representing 50–60% of a basin’s fish assemblage (Figure 7). By contrast, equilibrium
strategists were relatively more prevalent in the
United States compared to Australia, whereas
periodic strategists were more common in
Australia compared to the United States. Both
approaches to quantifying life history compo-
98
olden and kennard
Figure 6. Two-dimensional ordination plot resulting from the principal component analysis on the
seven life history traits for the fish species pools of the United States and Australia. Species are identified by solid symbols (United States) and open symbols (Australia). Traits with the highest combined
loadings (>0.50) on the first two principal components are illustrated. Full descriptions of trait abbreviations are in Table 1.
sition of the drainage basins produced similar
results. The relationship between proportional
strategy richness and distance-weighted proportional strategy richness was significantly
positive for periodic (Pearson’s R = 0.97, P <
0.001), opportunistic (R = 0.76, P < 0.001),
and equilibrium strategies (R = 0.93, P <
0.001). Given the correlation between these
estimates of life history composition (and similar results from multiple regression analyses),
we report on the results based on the distanceweighted proportional strategy richness.
Results from the multiple regression analysis provided strong support for the influence of
hydrologic variability on fish life history trait
composition in the United States and Australia
(Table 2). In agreement with predictions from
life history theory, hydrologic variability for
both countries was positively (and significantly) associated with the prevalence of opportunistic strategists and negatively associated with
periodic strategists. We found no statistical
difference in the slope of these associations
between the two continents. In contrast to
life history theory, increasing hydrologic variability in Australia (but not the United States)
was related to greater proportions of equilibrium strategists. Net primary productivity was
positively concordant with the proportion of
periodic strategists in Australian basins and
with the proportion of opportunistic strategists in the United States and was negatively
concordant with the equilibrium strategy for
both countries, supporting predictions from
life history theory. One exception, however,
was the significant negative relationship between NPP and the proportion of species considered opportunistic in Australia. In contrast
to flow variability, the slopes of this relationship differed for opportunistic and periodic
strategies but were not significant different for
equilibrium strategies (Table 2). These results
intercontinental comparison of fish life histories
99
Figure 7. Ternary plot illustrating the relative proportion of periodic, opportunistic and equilibrium strategists in drainage basins of the United States (n = 56, solid symbol) and Australia (n = 56, open symbol).
show that the association between hydrologic
variability and life history of fishes was similar
in the United States and Australia (two out of
three strategies based on directionality and significance) but weaker concordance with productivity (one out of three strategies).
Discussion
Riverine biodiversity is generated and maintained by geographic variation in stream processes and fluvial disturbance regimes, which
largely reflects regional differences in climate,
geology, and topography. When investigating
fish community–environment relationships
at large spatial scales, a functional perspective
using biological traits allows the comparison
of species compositions that naturally differ
due to biogeographic constraints on regional
species pools. This is particularly relevant for
our study of the fish faunas of southern United
States and eastern Australia, which only have
a single species/genus/family in common,
flathead mullet Mugil cephalus. Despite widely
different biogeographic histories, our study
found that patterns of life history composition
of river basins responded similarly along a gradient of hydrologic variability.
In support of previous research for freshwater fishes in a diversity of environments
(Winemiller 1989; Winemiller and Rose 1992;
Vila-Gispert and Moreno-Amich 2002; VilaGispert et al. 2002; Olden et al. 2006, Tedesco
et al. 2008), the fishes of the United States and
Australia examined in our study conform to
the three-dimensional adaptive space arising
from the interrelationships among three basic
demographic parameters of survival, fecundi-
0.115
0.476
*
1.034
<0.001
*
0.391
0.048
*
–0.418
0.042
R2 = 0.303, F4,51 = 5.57, P < 0.001
–0.443
0.006
*
–1.134
<0.001
*
–0.605
0.002
*
0.498
0.013
R2 = 0.358, F4,51 = 7.10, P < 0.001
0.642
<0.001
0.720
0.001
*
0.585
0.001
–0.361
0.043
*
R2 = 0.478, F4,51 = 11.66, P < 0.001
Model: Periodic Strategy
–0.077
0.524
Basin area (km2)
Runoff (ML/km2)
–1.140
0.005
Flow variability
–
–1.200
<0.001
NPP (metric tons of carbon)
+
–0.300
0.181
Model
R2 = 0.332, F4,51 = 6.34, P < 0.001
Model: Equilibrium Strategy
–0.051
0.695
Basin area (km2)
Runoff (ML/km2)
0.579
0.174
Flow variability
–
–0.257
0.449
NPP (metric tons of carbon)
–
–0.631
0.011
Model
R2 = 0.225, F4,51 = 3.70, P = 0.010
Comparison
of estimates
0.091
0.444
0.637
0.105
1.137
<0.001
0.589
0.010
R2 = 0.346, F4,51 = 6.74, P < 0.001
Basin area (km2)
Runoff (ML/km2)
Flow variability
+
NPP (metric tons of carbon)
+
Model
Model: Opportunistic Strategy United States
Australia
Source of variation
Prediction
Estimate
P
Estimate
P
Table 2. Results from the multiple regression models relating proportional life-history strategy richness to basin area (log-transformed), annual runoff
(log-transformed), flow variability (coefficient of variation) and annual net primary productivity (NPP, log-transformed). Life-history theory predictions
of the factors associated with endpoint strategies are presented and follow Winemiller (1995, 2005) and Figure 1. * indicates no significant difference
of regression estimates (slopes) between the United States and Australia based on P < 0.05, thus providing support for community convergence in
life-history composition.
100
olden and kennard
intercontinental comparison of fish life histories
ty, and onset and duration of reproductive life.
Our analyses revealed that species from both
countries represent the endpoints defining the
periodic, opportunistic, and equilibrium life
history strategies, as well as occupying intermediate positions in life history space. A large
majority of American fishes were characterized
by the suite of opportunistic attributes, including small body size, short-lived, and low batch
fecundity with multiple reproductive batches
per breeding season. By contrast, the majority of
Australian fishes were located along a life history
axis connecting the opportunistic endpoint to
the midpoint of the edge attaching the periodic
and equilibrium endpoints. This axis defines a
gradient of evolutionary “bet-hedging” (sensu
Cole 1954) considered adaptive in relatively
unpredictable environments where conditions
are occasionally so bad that recruitment fails
entirely (Stearns 1992). The bet-hedging suite
of traits is also associated with relatively greater
mobility to promote rapid recolonization after
disturbance, a pattern supported by our finding that Australian fishes are characterized by
smaller swim factors compared to fishes of the
United States, indicating a stronger swimming
ability (Webb 1984). Last, we found that approximately three-quarters of fishes in the United States exhibited relatively more parental care
by hiding or guarding their eggs during reproduction, compared to Australia where approximately two-thirds of the species spawn on open
substrate and exhibit no egg guarding.
Our study, together with other largescale scale comparisons of freshwater fish
traits, provides strong evidence that essential
trade-offs produce intercontinental similarities in life history strategies despite divergent
phylogenies. For example, in the edited book
Community and Evolutionary Ecology of North
American Stream Fishes, Moyle and Herbold
(1987) reported that the body size and life history characteristics of fish communities in cold
101
headwater streams are very similar in North
America and Europe, despite their independent origins. Through a series of exploratory
analyses, the authors proposed that fish faunas
in Europe and western North America shared
greater trait similarity compared to eastern and
western fishes of North America. Research by
Winemiller and colleagues have showed that
the freshwater fishes of South America and
North America both encompass the trilateral
life history space defined by the opportunisticperiodic-equilibrium trichotomy (Winemiller
1989; Winemiller and Rose 1992). This association was further explored by Vila-Gispert
et al. (2002) who found consistency in basic
life history patterns among 301 fish species
from different regions of Europe, North America, and South America. Species from South
America showed a greater tendency toward the
opportunistic suite of traits, whereas North
American and European fishes occupied immediate positions along an axis linking the opportunistic and periodic strategies.
Drawing definitive conclusions from
this study is complicated by the fact that fish
species data were collected from entire continents, thus ignoring spatial variation in environmental conditions that shape species
traits and assemblage functional composition. The present study tackles this problem
by comparing fish faunas from regions experiencing similar long-term climatic regimes.
Based on a hard classification of species, we
found that the southern United States and
eastern Australia contained similar fractions
of equilibrium species (12% versus 15%, respectively), whereas the United States contained a greater proportion of opportunistic
species (69% versus 52%) but less periodic
species (19% versus 33%). However, it is important to note that the majority of species in
both continents exhibit immediate strategies
between these extremes.
102
olden and kennard
We found that hydrologic variability and
regional climate were related to patterns of fish
life history variation in the United States and
Australia. These relationships were generally
the same between countries and consistent
with predictions from life history theory for
hydrologic variability but were much more
variable for NPP. Hydrologic variability (both
countries) and NPP (United States only) were
positively related to basins with higher prevalence of opportunistic strategists, a life history
that should maximize fitness in environmental
settings dominated by unpredictable environmental change that cause frequent densityindependent mortality, followed by rapid
colonization (Winemiller 1995, 2005). Many
opportunistic species in southern United States
and eastern Australia are associated with shallow riffle and marginal habitats, where changes
in hydrologic regimes, including floods and
droughts, induce major alterations in water
depth, substrate characteristics, and habitat
availability (Hoeinghaus et al. 2007; Kennard
et al. 2007). In the absence of intense predation and resource limitation (likely to exist in
warmer, productive regions) that promote low
juvenile density dependence, opportunistictype species should be favored due to their
high turnover (short juvenile phase of the life
cycle) and high annual fecundity (multiple
spawning events) (McCann 1998).
Our findings also provide strong support
for the negative influence of hydrologic variability on the prevalence of periodic strategists
in basins of both the United States and Australia. The periodic strategy maximizes fecundity by delaying maturity (to achieve sufficient
size and to acquire resources) and producing
many smaller eggs as a tactic to increase juvenile survivorship. According to life history
theory, large body size of periodic strategists
enhances adult survivorship during suboptimal conditions and permits storage of energy
and nutrients for future bouts of reproduction (Winemiller and Rose 1992). Because
freshwater environments exhibit either spatial
or temporal variability that is to some degree
predictable, the periodic production schedule
allows a fitness payoff when reproduction coincides with favorable environmental conditions. Given that drought and flood conditions
represent significant agents of physical change
in freshwater ecosystems (Lake 2008), small,
but frequent, extreme flow events that induce
mortality of early life stages of periodic-type
species may significantly influence population
abundance and species distributions.
In contrast to life history theory, we found
no support for the predicted negative relationship between hydrologic variability and the
distribution of equilibrium strategists in Australia (a negative but nonsignificant relationship was observed for the United States). The
equilibrium strategy optimizes juvenile survivorship by appointing a greater amount of material into each individual egg and/or the provision of parental care (Winemiller and Rose
1992). However, in support of theory—which
predicts that the equilibrium configuration of
life history traits should be favored in high juvenile density-dependent and resource-limited
environments (McCann 1998)—we observed
a significant negative relationship between
net primary productivity and the prevalence
of equilibrium species in basins of both the
United States and Australia. This indicates that
freshwater ecosystems located in less productive watersheds are characterized by relatively
more equilibrium-type species. It is important
to note that in contrast to hydrologic variability, there are strong predictions of how equilibrium-periodic life history strategies respond to
patterns of hydrologic seasonality (i.e., predictability). For example, in support of theory, Tedesco et al. (2008) found a higher proportion
of periodic strategists in highly seasonal basins
intercontinental comparison of fish life histories
of West Africa that exhibit short and predictable hydrologic season. The quantification of
flow regime predictability in our study regions
is presently not feasible, but will be the subject
of future investigation.
We found moderate evidence that life history traits of freshwater fish communities in
southern United States and eastern Australia have converged across similar gradients of
hydrologic variability (but not productivity)
despite phylogenetic and historical differences
between continents. Increasing hydrologic
variability has promoted the greater prevalence
of opportunistic strategists (and their associated biological traits) while concurrently minimizing the persistence of periodic-type species. Notably, no such relationship was found
for equilibrium strategists. Our findings at the
scale of the drainage basin correspond to the
results of Lamouroux et al. (2002) at the local
reach scale. Lamouroux et al. (2002) found
that, within continents, local hydraulic and geomorphic variables were highly correlated with
fish trait proportions in streams of France and
the United States (Virginia). Taken together,
convergence in the life history characteristics
of fish assemblages across biogeographically
distinct regions of the United States and Australia may imply the existence of key, repeated
evolutionary mechanisms responsible for the
observed flow:trait relationships.
Despite the role of hydrologic variability
and productivity in shaping life history diversity in Australia and the United States, divergent
life history strategies are still expected in locations experiencing the same long-term environmental regimes. This occurs because species
perceive the same environment very differently
from another and thus use their environment
at a range of different spatial and temporal
scales (Winemiller 2005). Moreover, species
“assigned” to the same life history strategy may
have similar functional relations to their envi-
103
ronment, yet the actual traits possessed by species may differ because they originate from different systematic groups (Verberk et al. 2008).
Consequently, other aspects of functional and
life history variation should vary across dominant environmental gradients in freshwater
ecosystems. Previous studies have shown that
the stability of flow regimes is directly related
to taxonomic and trophic diversity (Horwitz
1978), whereby assemblages characterized by
generalists in hydrologically variable sites and
more specialist species in stable sites (Poff and
Allan 1995). Hoeinghaus et al. (2007) also reported the importance of hydrologic stability
in shaping the functional attributes of fish faunas in rivers of Texas (United States).
Fish life history strategies represent different solutions to particular ecological problems, thus providing a connection between
species traits and environmental gradients in
freshwater ecosystems. Despite the powerful
role that species traits have to play in ecological studies, it is important to recognize a number of limitations that are common to previous
studies, including our own. First, there exists
a substantial temporal mismatch between the
time period over which environmental variables were quantified (i.e., past century) and
the evolutionary time period over which fish
species have evolved and responded to environmental change (i.e., millennia). For this reason, it is perhaps not surprising that previous
studies have had difficulty linking fish traits to
descriptors of the physical environment. Second, we acknowledge that intraspecific variations in life history traits do exist at large-spatial scales and that assigning a single trait value
to a species may not appropriately represent
this variation. However, Blanck and Lamouroux (2007) found that species fecundity and
life history traits related to body length varied
more among species than between populations
of the same species for European freshwater
104
olden and kennard
fishes, suggesting that the relative position of
species in opportunistic-periodic-equilibrium
life history space is unlikely to be significantly
affected by intraspecific trait variation. Third,
data availability required us to quantify interannual hydrologic variability (i.e., variation
in mean annual discharge) rather than intraannual seasonal variability, which we might expect to play a larger role in shaping at life history composition of fish communities (Tedesco
et al. 2008). Fourth, our grain of analysis was
the river basin, which clearly encompasses a
great deal of environmental heterogeneity. Future investigations that link river hydrology to
reach-scale fish assemblages are likely to yield
greater insight into inter-continental community convergence in life history composition.
In conclusion, balancing human and ecosystem needs for freshwater will require that
our view of the ecology of rivers and the impacts associated with water use and infrastructural development be placed in the context of
the natural flow regime and deviations from the
natural condition. Our study provides a conceptual framework of management options for
species in regulated rivers because life history
strategies are the underlying determinants for
population responses to environmental change
(King and McFarlane 2003; Winemiller 2005)
and therefore can be used to classify typical
population responses to flow alteration or mitigation via environmental flow prescriptions.
This can provide vital management advice for
defining flow targets and predicting fish species responses to flow restoration. Given that
hydrologic variability (this study) and predictability (Tedesco et al. 2008) act as landscape
filters for fish life history traits, we expect that
dam management practices will have persistent effects on fish faunas at local and regional
scales. In the long term, dam-induced dampening of flow variability resulting from operations
for flood control and water supply may shift
communities away from opportunistic strategies and toward more periodic-equilibrium
forms (Olden et al. 2006).
Acknowledgments
We gratefully acknowledge Meryl Mims and
two anonymous reviewers for helpful comments, Zachary Shattuck for helping populate
the fish trait database, Janet Stein for provision
of environmental data, and Tarmo Raadik and
Tom Raynor for providing unpublished fish
distributional data. Funding support was provided by the USGS Lower Colorado River Basin Aquatic GAP Program ( JDO) and a postdoctoral fellowship from the Australian Rivers
Institute, Griffith University (MJK). JDO and
MJK conceived and developed the idea for the
manuscript and assembled the data sets, JDO
conducted the data analysis, and JDO and
MJK wrote the manuscript.
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